1. Field of Invention
The present invention relates to an LCD panel, and more particularly to a thin-film-transistor (TFT) array panel structure and a fabrication method thereof.
2. Related Art
TFTs are widely used in display pixel drive of active matrix LCDs. In general, the method of fabricating a TFT array panel of an active matrix LCD must include five mask processes. Firstly, a first metal layer is deposited on an insulating substrate (for example, a glass substrate) which serves as a display panel, and a gate electrode, a lower electrode of a storage capacitor, a scan line, and a scan line pad are formed with a first mask. Then, a gate insulator layer, an a-Si layer, and an N+ a-Si layer are sequentially deposited on the first metal layer, and the a-Si layer and N+ a-Si layer are defined as TFT islands with a second mask. Thereafter, a second metal layer is deposited on the N+ a-Si layer and the gate insulator layer. Next, a channel is formed with a third mask and back-channel etching (BCE), so as to define a pair of drain/source electrodes, an upper electrode of a storage capacitor, a data line, a data line pad, and a pixel display region. Afterward, a passivation layer, for example, a nitride is deposited to protect the transistor and the storage capacitor structure, the pattern of the passivation layer is defined with a fourth mask, and a contact hole is formed. Finally, a transparent electrode layer, for example, indium tin oxide (ITO) is deposited on the passivation layer, and the pattern of a pixel electrode is defined with a fifth mask. The drain electrode is electrically connected to the pixel electrode via the contact hole formed by the passivation layer.
To reduce the number of the masks used, U.S. Patent Publication U.S. Pat. No. 4,624,737 proposes a fabrication method of TFT, which includes the following steps. (1) Form a gate electrode on an insulating substrate. (2) Forming a gate insulator layer, a high-impedance semiconductor layer, a low-impedance semiconductor layer, and a conductor layer sequentially on the substrate, and etching the high-impedance semiconductor layer, low-impedance semiconductor layer, and conductor layer to form an island region thereabove. (3) Etching the low-impedance semiconductor layer and the conductor layer, such that the two layers are fractured to form a source electrode and a drain electrode respectively. (4) Forming a passivation layer and etching the passivation layer, such that the passivation layer covers the source electrode and drain electrode of the TFT.
The aforementioned transistor structure can be applied in a transmissive LCD or a reflective LCD. When applied in a transmissive TFT LCD panel, the conductor layer of the transistor structure must adopt materials such as ITO as transmissive conductors, i.e., the data line of the transmissive LCD is formed by ITO. As the impedance of the ITO is higher than a general metal conductor, the signal is easily distorted when transmitted through the data line, and thus the operating frequency of the pixel of the LCD panel is negatively affected and the response rate cannot be easily accelerated. Moreover, when the aforementioned transistor structure is applied in a transmissive LCD or a reflective LCD, a suitable metal conductor must be selected to avoid the poor bonding when the pads of the LCD are bonded to other electronic components.
Further, in the aforementioned method of fabricating TFT array panel with five masks, the second metal layer serving as the upper electrode of a storage capacitor is made of an opaque material. In process, in order to overlap the second metal layer and the first metal layer to form a storage capacitor structure, based on the consideration of process tolerance, the second metal layer of the upper electrode of the storage capacitor is larger in area than the first metal layer of the lower electrode, which usually results in a low aperture ratio.